1. Field of the Present Invention
The present invention relates to a semiconductor device having a memory element utilizing a silicide reaction and a method for manufacturing the same.
2. Description of the Related Art
Various types of memory have been proposed in the past. As typical memory, memory including an electromagnetic tape or an electromagnetic disk, a RAM capable of writing and reading, a ROM only for reading (read only memory), and the like can be given.
As a conventional ROM, the following can be given: a mask ROM which stores information using a mask in a process for manufacturing an IC, a fuse ROM which stores information by melting down a fuse element with current after manufacturing an IC chip, an antifuse ROM which stores information by short-circuiting an insulator with current after manufacturing an IC chip, and the like.
Since the mask ROM stores information using a mask in a process for manufacturing an IC, a mask corresponding to information to be written has to be prepared, and thus, manufacturing cost has been increased. In addition, the fuse ROM has had a possibility of causing malfunction due to dust generated when a fuse element is melted down.
In addition, the antifuse ROM is more advantageous than other ROM in that the antifuse ROM does not need a mask corresponding to information to be written in manufacturing and dust is not generated when information is written to the memory. In recent years, a technique for forming the antifuse ROM over a silicon substrate by utilizing a silicide reaction has been proposed (for example, Patent Document 1: Japanese Patent No. 3501416).
An anti-fuse ROM disclosed in the Patent Document 1 has a plurality of memory elements including a pair of conductive film serving as a cathode or anode and an amorphous silicon film interposed between the conductive films. The memory elements are elements utilizing variation in resistance of the memory elements in which the conductive films serving as a cathode or anode and the amorphous silicon film take a silicide reaction by application of voltage between the cathode and anode.
The antifuse ROM, for example as shown in FIG. 12A, includes a wiring 1202 functioning as a word line, which is formed over a semiconductor substrate 1201, a amorphous silicon film 1203 formed over the wiring 1202 functioning as a word line, and a wiring 1204 functioning as a bit line, which is formed over the amorphous silicon film 1203. The amorphous silicon film 1203 is sandwiched between the wiring 1202 functioning as a word line and the wiring 1204 functioning as a bit line. In this way, it is effective to form the amorphous silicon film at an intersection of the word and bit lines in miniaturization of the memory elements.
In recent years, a semiconductor device with a wireless communication function, specifically a wireless chip, has been expected to have a large market, thereby having attracted attention. Such a wireless chip is referred to as an ID tag, an IC tag, an IC chip, an RF (radio frequency) tag, a wireless tag, an electronic tag, and an RFID (radio frequency identification) in accordance with usage.
A wireless chip includes an interface, memory, a control portion, and the like. As the memory, RAM capable of writing and reading and a ROM only for reading are used, and they are properly used in accordance with purposes. Specifically, a memory region is assigned for each particular application, and an access right is managed for each application and each directory. In order to manage the access right, the wireless chip has a verification means which compares and verifies a private code of applications and a control unit which gives users the access right of the applications with which the private code is identical, as a result of the comparison and verification by the verification means. Such a wireless chip is formed using a silicon wafer, and integrated circuits such as a memory circuit and an arithmetic circuit are integrated over a semiconductor substrate.
When a card on which such a wireless chip is mounted (a so-called IC card) is compared to a magnetic card, an IC card has advantages of having a large memory capacity, an arithmetic function, high authentication accuracy, and great difficulty in falsification. Therefore, an IC card is suitable for management of personal information. A ROM only for reading is commonly used as memory mounted on an IC card so that falsification is impossible.